1. Field of the Invention
The present invention relates to the use of a particular family of complexing agents and, more specifically, diglycolamides, for increasing the separation factor between americium and curium and/or lanthanides in a liquid-liquid extraction operation.
The invention may find application in the field of processing and recycling irradiated nuclear fuels where it has a most particular advantage for selectively recovering americium from aqueous solutions with high activity such as for example raffinates stemming from the processing of irradiated nuclear fuels by a PUREX or COEX™ method.
It may also find application in the field of the processing of rare earth ores for example of the monazite, xenotime or bastnaesite type, in order to facilitate the separation of <<lightweight>> rare earths, i.e. with an atomic number of less than 63 (lanthanum, cerium, praseodymium, neodymium, samarium), of <<heavy>> rare earths, i.e. with an atomic number of more than 63 (europium, gadolinium, terbium, . . . ), and optionally yttrium which may be recovered pure, or the one of two rare earths with adjacent or close atomic numbers such as neodymium and samarium. Pure yttrium has many applications such as luminescence, fluorescence and optical materials.
2. Description of the Related Art
The methods, with which uranium and plutonium present in the dissolution liquors of irradiated nuclear fuels may be extracted and purified, such as the PUREX method (which is presently used in factories for processing irradiated nuclear fuels) and the COEX™ method (which is described in PCT International Application WO 2007/135178, [1]), generate effluents to which the name of raffinates is given.
These raffinates are aqueous solutions with strong nitric acidity, typically from 2 to 5 M, which contain two minor actinides, i.e. americium and curium, lanthanides such as lanthanum, cerium, praseodymium, neodymium, samarium and europium, fission products other than lanthanides such as molybdenum, zirconium, rubidium, ruthenium, rhodium, palladium and yttrium, as well as corrosion products such as iron and chromium.
Their handling presently consists of concentrating them as much as possible and then packaging them in glassy matrixes with view to storing them before ultimate storage.
Since the beginning of the 90s, research has actively been conducted in France for achieving extensive separation of metal elements present in the raffinates stemming from the processing of irradiated nuclear fuels by the PUREX method. The main goal of this extensive separation is to reduce the radiotoxicity of the vitrified waste by removing from these raffinates the most radiotoxic elements with the purpose of transmuting them.
An extensive separation route was particularly explored: this is hydrometallurgy which is based on separation methods by liquid-liquid extraction.
Now, it is found that most extractants, the use of which has been suggested to this day such as diisobutylphenyloctylcarbamoylmethylphosphine (or CMPO) and other carbamoylphosphine oxides, trioctylphosphine oxide (or TOPO) and other phosphine oxides, N,N′-dimethyl-N,N′-dioctylhexylethoxymalonamide (or DMDOHEMA) and other malonamides, diisodecylphosphoric acid (or DIDPA) and other phosphoric acids, etc., has a not very marked selectivity, or even no selectivity at all, for americium with respect to curium. This is due to the very large similarity existing between the physicochemical properties of both of these elements.
The result of this is that it is presently extremely difficult to separate americium from curium by liquid-liquid extraction and that this separation, when it is possible, requires in order to obtain satisfactory separation performances, the use of a large number of stages, which is a penalty from the industrial point of view.
Now, it would be highly interesting to be able to selectively recover the americium present in the raffinates stemming from the processing or irradiated nuclear fuels by the methods of the PUREX or COEX™ type, before sending these raffinates to vitrification.
Indeed, in addition to reducing the radiotoxicity of vitrified waste, selective recovery of americium would allow significant lowering of the thermal load of this waste, and thereby their storage footprint.
Moreover, curium 244, which represents the majority isotope of the curium present in nuclear waste, is a powerful neutron emitter, a source of significant radioactivity. Recovering americium without curium would therefore allow considerable simplification in the manufacturing, handling and transport of transmutation fuel assemblies containing americium. Transmutation nuclear fuels may thereby contain more americium.
The Inventors therefore set their goal to finding a means which would generally allow facilitation of the separation of americium and of curium when it is sought to separate both of these elements from each other by liquid-liquid extraction.
They also set their goal that this means should be efficient and therefore usable both in the case when it is sought to separate americium from curium from an aqueous solution containing, in addition to both of these elements, lanthanides and other fission products and in the case when it is sought to separate americium from curium from an aqueous solution only containing both of these elements. In this perspective, they notably desired that this means should not set into play any oxidation-reduction reaction of americium or of any other metal element.
Finally, they set their goal that this means should be simple to apply and that its use at an industrial scale, downstream from a PUREX or COEX™ method, should not add to the constraints already imposed by these methods, additional constraints, notably in terms of handling of the generated effluents, of corrosion, toxicity and safety.
Now, it turns out that within the scope of their work, the Inventors noticed that the presence of a diglycolamide in an acid aqueous phase containing americium, curium, lanthanides and other fission products, when this aqueous phase is subject to a liquid-liquid extraction operation, i.e. when it is put into contact with an organic phase containing an extractant (other than a diglycolamide) and then separated from this phase, is expressed by a substantial increase in the separation factor between americium and curium as compared with the one obtained in the absence of diglycolamide and this, regardless of the selectivity level which the extractant contained in the organic phase has for americium with respect to curium.
Further they noticed that the presence of the diglycolamide in said acid aqueous phase is also expressed by an increase in the separation factor between <<lightweight>> lanthanides and <<heavy>> lanthanides as compared with that obtained in the absence of the diglycolamide.
These observations are at the basis of the present invention.
It should be noted that the use of diglycolamides and in particular of N,N,N′,N′-tetra-octyldiglycolamide (or TODGA), is known, either alone or associated with other compounds of the tri-n-butyl phosphate (or TBP) or N,N-dihexyloctanamide (or DHOA) type, as extractants for extracting from an aqueous phase either lanthanides, or lanthanides and actinides(III), or lanthanides and actinides(III) and (IV), or further the whole of the actinides(III), (IV), (V) and (VI) (see for example ANSARI et al., Separation and Purification Technology, 66, 118-124, 2009 [2]; SHIMOJO et al., Dalton Transactions, 37, 5083-5088, 2008 [3]; MODOLO et al., Solvent Extraction Ion Exchange, 25, 703-720, 2007 [4]; French patent application 2 810 679 [5]; ANSARI et al., Separation and Purification Technology, 63, 239-242, 2008 [6]; Japanese patent application 2005-114448 [7] and French patent application 2 907 346 [8]).
The use of diglycolamides as complexing agents is also known for stripping from an organic phase, actinides, lanthanides and other fission products (see, for example SASAKI et al., Analytical Sciences, 23, 727-731, 2007 [9]).
On the other hand, what has never been proposed in the literature, is the use of diglycolamides as complexing agents for increasing the separation factor between americium and curium and/or between lanthanides in a liquid-liquid extraction operation.